So, rounser, you are saying that T1-4 (Temple of Elemental Evil), G1-3 (Against the Giants), D1-2 & Q1 (Descent – Demonweb Pits), A1-4 (Slavers), U1-3 (Saltmarsh), etc. are all railroad adventures?
Yes. The vast majority of module series created for pencil and paper D&D are railroads, because you have no real choice as to "what dungeon to sack next". The "next part" of the campaign arc is assumed to be the next slaver module, or the temple after the moathouse, and PCs don't change that. I can't remember whether the Saltmarsh series is slightly more flexible in this regard, but it's still "next dungeon" from memory.
Ironically, one of the best examples of a non-railroad module is
Ruins of Adventure, based on the
Pool of Radiance computer game. The players (shock, horror) get to choose which city block they're going to raid next, or whether they're going to go attack wilderness lairs, or deliver a package to a nearby fort etc. For D&D modules, this is really unusual - the "next dungeon" is usually non-negotiable.
I think that this is because railroading is so built into pencil and paper D&D in a "you get what you're given" way (just look at this thread), and computer gamers don't put up with such shackles to the same degree in their FRPGs. Why should they? Instead of one DM, a whole bunch of game designers can make their world big and flexible, and with no word count to worry about the amount of material can be exponentially bigger than that in a published module.
What I think you're failing to understand about what I'm getting at Quasqueton is that although railroading isn't ideal, it doesn't necessarily mean there's no fun to be had. My point is that a campaign arc can be improved by
not forcing the players in this way, and actually giving them some control over the course of the campaign, although as I've noted earlier this can be a lot more challenging for the DM, and would balloon out word count if done in module form, so generally isn't done...and so things are as they are now.
Railroading also solves a design problem for D&D: What if the PCs go to the Valhingen Graveyard whilst they're too low level to handle it?
Ruins of Adventure lets them get thumped until they die or retreat. With the advent of 3E, something like a lock that only a mid-level character could open might help keep lower level PCs out, but arguably this is a "choice killer" of it's own. It's an interesting topic to me, largely because the game seems to sidestep or handwave it.